Thermal imaging or thermography is a recording process wherein images are generated by the use of thermal energy. In thermography three approaches are known:
1. Direct thermal formation of a visible image pattern by image-wise heating of a recording material containing matter that by chemical or physical process changes colour or optical density. PA1 2. Image-wise transfer of an ingredient necessary for the chemical or physical process bringing about changes in colour or optical density to a receptor element. PA1 3. Thermal dye transfer printing wherein a visible image pattern is formed by transfer of a coloured species from an image-wise heated donor element onto a receptor element.
Direct thermal thermography is concerned with materials which are substantially not photosensitive, but are sensitive to heat or thermosensitive. Image-wise applied heat is sufficient to bring about a visible change in a thermosensitive imaging material. Most of the "direct" thermographic recording materials are of the chemical type. On heating to a certain conversion temperature, an irreversible chemical reaction takes place and a coloured image is produced. This irreversible reaction can be, for example, the reaction of a leucobase with an acid to produce the corresponding dye or the reduction of an organic or inorganic metal compound (e.g. silver, gold, copper or iron compounds) to its corresponding metal thereby producing a visible image. Such imaging materials are described, for example, in U.S. Pat. No. 3,080,254, EP-B 614 770, EP-B 614 769, EP-A 685 760, U.S. Pat. No. 5,527,757, EP-A 680 833, U.S. Pat. No. 5,536,696, EP-B 669 876, EP-A 692 391, U.S. Pat. No. 5,527,758, EP-A 692 733, U.S. Pat. No. 5,547,914, EP-A 730 196 and EP-A 704 318. A survey of "direct thermal" imaging methods is given e.g. in the book "Imaging Systems" by Kurt I. Jacobson-Ralph E. Jacobson, The Focal Press--London and New York (1976), Chapter VII under the heading "7.1 Thermography". According to U.S. Pat. No. 3,080,254 a typical heat-sensitive copy paper includes in the heat-sensitive layer a thermoplastic binder, e.g. ethyl cellulose, a water-insoluble silver salt, e.g. silver stearate and an appropriate organic reducing agent, of which 4-methoxy-1-hydroxy-dihydronaphthalene is a representative. Localized heating of the sheet in the thermographic reproduction process, or for test purposes by momentary contact with a metal test bar heated to a suitable conversion temperature in the range of about 90-150.degree. C., causes a visible change to occur in the heat-sensitive layer. The initially white or lightly coloured layer is darkened to a brownish appearance at the heated area. In order to obtain a more neutral colour tone a heterocyclic organic toning agent such as phthalazinone is added to the composition of the heat-sensitive layer. Thermosensitive copying paper is used in "front-printing" or "back-printing" using infra-red radiation absorbed and transformed into heat in contacting infra-red light absorbing image areas of an original as illustrated in FIGS. 1 and 2 of U.S. Pat. No. 3,074,809.
EP-B 726 852 discloses a recording material comprising on the same side of a support, called the heat-sensitive side, (1) one or more layers comprising an imaging composition essentially consisting of (i) a substantially light-insensitive organic silver salt being in thermal working relationship with (ii) a reducing agent, and (2) at the same side covering the imaging composition a protective layer, characterized in that the protective layer mainly comprises a cured polymer or cured polymer composition.
WO 94/16905 discloses a thermographic recording film comprising a support carrying: (a) an image-forming system; and (b) a protective layer comprising at least two epoxide moieties in the protective layer topcoat layer and/or in a layer on top of the protective topcoat layer, the ratio of colloidal silica to the compound containing at least two epoxide moieties being at least 2:1 by weight.
Thermographic recording materials with protective layers according to the teaching of EP-B 726 852 incorporating the imaging composition thereof caused premature failure of thermal heads. O. P. Srivastava suggested at the 3rd Annual Printing Workshop held between 23 and 25 March 1992 in Cambridge, Mass., U.S.A., that sodium and/or potassium ions in thermographic materials form their hydroxides with water present in the atmosphere during thermal printing and that these hydroxides dissolve the outermost glass coating of thermal printing heads and then migrate into the resistor material accelerating heating element failure. Reduction of the sodium and potassium ion concentration to below 601 ppm as disclosed in U.S. Pat. No. 4,396,684 was of itself insufficient to ensure that a protective (outermost) layer, in contact with a thermal head and covering an imaging layer, performed its function of protecting the imaging composition from damage and distortion without causing premature failure of thermal heads and in particular thin film thermal heads. There is therefore a need for thermographic recording materials with outermost layers, in contact with a thermal head and covering an imaging layer thereof, which perform the function of protecting the imaging composition from damage and distortion without causing premature failure of thermal heads.
Surprisingly it has been found that thermographic recording materials having outermost layers according to the teaching of EP-B 726 852 with combined sodium and potassium ion concentrations of less than 601 ppm did not necessarily provide protection to the imaging layers thereof. The use in such outermost layers of colloidal silica with free acid groups produced thermographic recording materials which upon image-wise heating with a thermal head became severely distorted and physically damaged. In addition there was an undesirable build-up of material on the thermal head.